1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. Steps to reduce automotive pollutants while maintaining performance and gasoline quality, increasing the required knock resistance of gasoline components as measured by gasoline "octane" number, have been a major factor in the growth of catalytic-reforming capacity and continue this trend in many areas of the world. The market for petrochemicals derived from gasoline-range aromatics continues to grow substantially, creating a need for incremental reforming capacity, severity and/or efficiency. Many producers of aromatics are looking for ways to use or upgrade existing reforming capacity through more effective reforming processes and catalysts in order to meet this incremental need without building expensive new catalytic-reforming process units.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke. In this manner, low-value paraffinic raffinates as well as naphthas can be upgraded to valuable aromatics.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed by a number of companies active in technology development. Commercialization of this dehydrocyclization technology nevertheless has been slow, probably due at least in part to the intolerance of the such catalysts to contaminants such a sulfur, nitrogen and condensed hydrocarbons. Paraffinic raffinates from aromatics extraction, which are particularly suitable for upgrading with zeolitic catalysts, require special attention relating to specific contaminants as addressed in the present application.
The harmful effects of fused multi-ring aromatic hydrocarbons on gasoline quality and coke formation in reforming are recognized in U.S. Pat. No. 4,664,777 (Hudson et al.). Feedstocks distilling at 177.degree. C. and above are converted catalytically in the presence of hydrogen to lower-boiling hydrocarbons. Elimination of fused multi-ring compounds, especially three-ring compounds, that could affect the noted gasoline endpoint specification of 225.degree. C. is particularly disclosed.
U.S. Pat. No. 4,804,457 (Ngan) teaches multistage adsorption of polynuclear aromatics after each of a series of reforming reactors to reduce coking rate and improve gasoline quality. Ngan addresses the processing of a feed having boiling range of 100.degree. to 400.degree. C., and the polynuclear aromatics removed preferably have three or more aromatic rings; the special contaminant problems associated with the processing of paraffinic raffinates are not addressed.